object and reference immutability using java generics

Object and Reference Immutability using Java Generics

Yoav Zibin(1), Alex Potanin(2), Shay Artzi(1), Adam Kiezun(1), and Michael D. Ernst(1)

1) MIT Computer Science and Artificial Intelligence Lab, USA2) Victoria University of Wellington, New-Zealand

Presenting: Ori Arad ([email protected]) – 14/2/07

2/23

Immutability – What for?...

Program comprehension

Verification Compile & Run-time

optimizations Invariant detection

Refactoring Test input

generation Regression oracle

creation Specification

mining Modeling

3/23

Immutability varieties

Class immutability No instance of an immutable class may be

change after creation (e.g. String, Integer etc.) Object immutability

Immutable instances may not be changed, while other instances (of the same class) may…

Reference immutability a given reference cannot be used to modify its

referent.

4/23

IGJ - Immutability Generic Java

Object immutability: An object: mutable or immutable.

Reference immutability: A reference: Immutable, Mutable, or

ReadOnly Class immutability

Also support Class immutability

5/23

IGJ - Immutability Generic Java

one new generic parameter (at the beginning of the list of generic parameters) was added

1: // An immutable reference to an immutable date; side effects are prohibited

2: Date<Immutable> immutD = new Date<Immutable>();3: // A mutable reference to a mutable date; side effects are permitted

4: Date<Mutable> mutD = new Date<Mutable>();5: // A readonly reference to any date; side effects are prohibited

6: Date<ReadOnly> roD = ... ? immutD : mutD;

1: // An immutable reference to an immutable date; side effects are prohibited

2: Date<Immutable> immutD = new Date<Immutable>();3: // A mutable reference to a mutable date; side effects are permitted

4: Date<Mutable> mutD = new Date<Mutable>();5: // A readonly reference to any date; side effects are prohibited

6: Date<ReadOnly> roD = ... ? immutD : mutD;

6/23

IGJ main design principles

Transitivity: The design must provide transitive (deep) immutability

Purely static: There should be no runtime representation for immutability

Polymorphism: It must be possible to abstract over immutability without code duplication

Simplicity: The design should not change Java's syntax and have a small set of typing rules

7/23

Type Hierarchies for IGJ

The type hierarchy for immutability parameters

The top of the type hierarchy for all other classes

8/23

IGJ Subtype hierarchy

// Demonstrates how to copy value of readonly roObj to mutable mutableObjObject<ReadOnly> roObj = ...;

List<Mutable,Object<Mutable>> l = new List<Mutable,Object<Mutable>>();

((List<Mutable,Object<ReadOnly>>) l).add(roObj);

Object<Mutable> mutableObj = l.get(0); // mutableObj equals roObj

// Demonstrates how to copy value of readonly roObj to mutable mutableObjObject<ReadOnly> roObj = ...;

List<Mutable,Object<Mutable>> l = new List<Mutable,Object<Mutable>>();

((List<Mutable,Object<ReadOnly>>) l).add(roObj);

Object<Mutable> mutableObj = l.get(0); // mutableObj equals roObj

Legend:L List, O object, R ReadOnlyIM Immutable, M Mutable

For example:L< R,O<M> > meansList<ReadOnly, Object<Mutable>>

9/23

Java Array Co-Variance Problem

Read from Array: OK!Write to Array: Problem!

// A is a single-element array of String.String[] a = new String[1];

// B is an array of ObjectObject[] b = a;

// Assign an Integer to b. This would be possible if b really were// an array of Object, but since it really is an array of String,// we will get a java.lang.ArrayStoreException.b[0] = new Integer (1);

// A is a single-element array of String.String[] a = new String[1];

// B is an array of ObjectObject[] b = a;

// Assign an Integer to b. This would be possible if b really were// an array of Object, but since it really is an array of String,// we will get a java.lang.ArrayStoreException.b[0] = new Integer (1);

Solution with IGJ – immutability promise us

only reading no Co-Variance problem

10/23

The Field Rule

legal iff I(o) = Mutable.Example:

Employee<ReadOnly> roE = ...;

roE.address = ...; // Compilation error!

Employee<ReadOnly> roE = ...;

roE.address = ...; // Compilation error!

o.someField = exp;

Typing is guaranteed by several rules.Let I(x) denote the immutability of x

Rule I: Field Assignment Rule

11/23

"Immutability" of Methods

4 new annotations: @ReadOnly, @Mutable, @Immutable and @AssignFields

Example:@Mutable void m()

{...this...} Here: I(this) is Mutable In general: in any method m, I(this)

is the same as I(m) “this” immutability depends on the

context

12/23

Reference-Immutability Rules

Method Invocation Rule:

(not necessarily I(o) = I(m) )

Employee<Mutable> o = ...;o.setAddress(...); // OK since I(o) = Mutable and I(setAddress) = Mutable

o.getAddress(); // OK since I(o) = Mutable and I(getAddress) = ReadOnly

((Employee<ReadOnly>) o).setAddress(...); // Compilation error!

Employee<Mutable> o = ...;o.setAddress(...); // OK since I(o) = Mutable and I(setAddress) = Mutable

o.getAddress(); // OK since I(o) = Mutable and I(getAddress) = ReadOnly

((Employee<ReadOnly>) o).setAddress(...); // Compilation error!

o.m(...) is legal if I(o) is subtype of I(m).

13/23

1: class Edge {

2: private long id;

3: public @AssignFields Edge(long id) { this.setId(id); }

4: public @AssignFields synchronized void setId(long id) {

5: this.id = id; }

6: public @ReadOnly synchronized long getId() { return id; }

7: public @Immutable long getIdImmutable() { return id; }

8: public static void print(Edge<ReadOnly> n) {... }

9: }

10: class Graph {

11: public Edge lastN;

12: public List<I,Edge> l;

13: public @AssignFields Graph(List<I,Edge> l) { this.l = l; }

14: public @Mutable void addEdge(Edge n) {

15: this.l.add(n); this.lastN = n; }

16: public @ReadOnly Edge getLast() { return this.lastN; }

17: public static <T extends ReadOnly>

18: Edge<T> findEdge(Graph<T> nl, long id) { ... }

19: }


2: private long id;



5: this.id = id; }




9: }










19: }

ExampleIGJ classes Edge and Graph, with the immutability parameters (and annotations, for this) underlined.

For now:Assume @Immutable is like @ReadOnlyAnd @AssignFields is like @Mutable

14/23

Example:

Line 5: the assignment into this.id is OK If this.id = …; was on line 6 – illegal… Line 3: OK due to Method Rule Line 8: static no annotation, Edge of any

immutability could be pass here…


2: private long id;



5: this.id = id; }




9: }


2: private long id;



5: this.id = id; }




9: }

15/23

Example:

Line 11: a class can pass its immutability parameter to its fields

Line 16: Also return type – no need for overloading Line 12: Transitivity - in an immutable Graph the

field l will contain an immutable list of immutable edges










19: }










19: }

16/23

Object Immutability & constructors

What annotation should CTOR be?... @Mutable? problem:

public @Mutable Graph(List<I,Edge> l) {

this.l = l;

this.addEdge( new Edge<Mutable>(0) ); // }

…

List<Immutable,Edge<Immutable>> imList = ...;

new Graph<Immutable>(imList); // An element was added to imList

public @Mutable Graph(List<I,Edge> l) {

this.l = l;

this.addEdge( new Edge<Mutable>(0) ); // }

…

List<Immutable,Edge<Immutable>> imList = ...;

new Graph<Immutable>(imList); // An element was added to imList

@Immutable? Guess not…

17/23

Object Immutability & constructors

Solution: Forth kind of reference immutability: AssignFields

Permit to perform limited side-effects without permitting modification of immutable objects @Mutable method can assign & mutate @AssignFields method can only assign

18/23

Immutability & Assignability

MyClass myObject = new MyClass();

myObject = anotherObject; Assignability

myObject.setField(4); immutability

19/23

Revised rules

Field Rule revised (relaxed): AssignField is not transitive…

Method Rule revised (restricted):

o.someField = ...; is legal if I(o) = Mutableor (I(o) = AssignFields and o=this)

o.m(...) is legal if I(o) is subtype of I(m)and (I(m) = AssignFields implies o=this)

20/23

Example:

Line 5: the assignment into this.id is still OK setId is annotated with @AssignFields I(this)=AssignFields

Line 3: I(this) = AssignFields & I(setId) = AssignFields


2: private long id;



5: this.id = id; }

6: public @ReadOnly synchronized long getId() { return id;}



9: }


2: private long id;



5: this.id = id; }

6: public @ReadOnly synchronized long getId() { return id;}



9: }

21/23

Example:

Line 13: If we will add the following code – it will be illegal in the revised rule (and legal in the old one)

public @mutable Graph(List<I,Edge> l) {

this.l = l;

this.l.get(0).setId(42); // }

public @mutable Graph(List<I,Edge> l) {

this.l = l;

this.l.get(0).setId(42); // }

New Rule:new SomeClass<X,…>(…) is legal only if

X = Mutable and this CTOR is not marked as @Mutable, or X = Imuutable and this CTOR is not marked as @Immutable (X = readOnly & X=AssignFileds are illegal)

22/23

Future Work

Plug-in for IDE (e.g. Eclipse) A WriteOnly immutability parameter @readable notation for field

class Vector {

@readable int size; ...

@None int size() { return size; }

@WriteOnly void add(T t) { ... }

@WriteOnly void removeLast() { ... }

@Mutable void remove(T t) { ... }

@ReadOnly void get(int index) { ... }

}

class Vector {

@readable int size; ...

@None int size() { return size; }

@WriteOnly void add(T t) { ... }

@WriteOnly void removeLast() { ... }

@Mutable void remove(T t) { ... }

@ReadOnly void get(int index) { ... }

}

23/23

Future Work (cont.)

Add default immutability

class Graphclass Graph

An alternative syntax (Java 7?...)

Runtime support (e.g. down-cast)

@immutable Document[@readonly]

new @mutable ArrayList<@immutable Edge>(...)

@immutable Document[@readonly]

new @mutable ArrayList<@immutable Edge>(...)

object and reference immutability using java generics

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